This invention relates to low carbon, aluminum killed, cold rolled, recrystallization annealed steel. More particularly, the invention relates to a steel for porcelain enameling having niobium carbide precipitates for elevating yield strength to at least 21 kg/mm.sup.2 after being strained at least 8% and heated to at least 815.degree. C.
Steel sheets for household appliances such as ranges, dishwashers, cooktops, washers and dryers are formed into parts, cleaned to remove dirt and lubricants, specially treated by acid pickling to remove rust and scale and to deposit elemental nickel and then coated with a base coating of porcelain enamel. The as-coated parts are dryed and then heated at temperatures of about 760.degree.-870.degree. C. for fusing the enamel. Forming of the part can produce a strain of as much as 20% in the steel. During the heating, critical grain growth may occur in the steel part with a simultaneous loss of yield strength. This loss of yield strength requires thicker base metal to resist breakage by stresses imposed during service. It also allows flexure of the formed part during subsequent handling or use causing the enamel to chip off.
The prior art sought to avoid this loss of yield strength by adding elements such as niobium, titanium, zirconium, boron and vanadium which combine with carbon and/or nitrogen, or elements such as copper, silicon, chromium, phosphorus or manganese which strengthen by solid solution hardening. U.S. Pat. No. 3,183,078 discloses the addition of 0.005-0.035 wt. % aluminum and 0.05-0.20 wt. % titanium to a vacuum degassed melt having less than 0.02 wt. % carbon. After annealing, the steel is nonaging because soluble nitrogen and carbon are combined with aluminum and titanium as stable compounds. This steel is costly to manufacture and does not resist loss of yield strength during porcelain enameling. Canadian patent 934,275 discloses the addition of 0.02-0.06 wt. % niobium and 0.006-0.015 wt. % nitrogen to a steel melt containing at least 0.02 wt. % carbon and at least 0.10 wt. % manganese. The steel was hot rolled using a coiling temperature less than 677.degree. C. and decarburized to less than 0.008% C during annealing. Total elongations of about 30% after annealing and yield strengths up to 398 MN/m.sup.2 were disclosed after straining and heating. This steel is strengthened by precipitation of NbN during annealing. This steel can not be easily made by continuous casting and also is very costly to manufacture. U.S. Pat. No. 3,333,987 discloses adding less than a stoichiometric amount of one or more of the carbide forming elements of titanium, niobium or zirconium to a melt having at least 0.04 wt. % carbon. The steel is decarburized during annealing to remove any soluble carbon not combined with the carbide stabilizing element. Furthermore, a normalizing heat treatment is needed to develop suitable properties for forming parts. Yield strengths generally less than 21 kg/mm.sup.2 are disclosed after straining and heating. U.S. Pat. No. 3,598,658 discloses an enameling steel containing 0.09 wt. % carbon, 0.19 wt. % manganese, 0.026 wt. % phosphorus, 0.04 wt. % copper, 0.03 wt. % vanadium, 0.04 wt. % chromium, 0.03 wt. % niobium and 0.05 wt. % titanium. The steel was decarburized to 0.003 wt. % carbon during annealing. High elevated temperature yield strengths are alleged.
As indicated above, many prior art workers have long attempted to develop cold rolled steels for porcelain enameling. However, they have been unsuccessful at developing an inexpensive high strength steel for porcelain enameling using conventional melting, hot rolling and annealing practices. Addition of precipitating hardening and/or nitride forming elements in stoichiometric quantity to a melt to produce high strength enameling steel is undesirable because of the added alloy cost. Vacuum decarburizing the liquid steel or special decarburizing annealing cycles to produce such a steel also are undesirable because of added processing time and cost. Vacuum decarburizing the liquid steel also is undesirable because the steel will not be fishscale resistant because insufficient iron carbide particles form in the steel during cooling after hot rolling. Accordingly, there remains a need for an inexpensive, high strength, recrystallization annealed steel for porcelain enameling. More particularly, there remains a need for such a steel produced using conventional processing.